Urinary prostaglandin E-like material (iPGE) was measured in the urine of dogs during control observations, during the last forty minutes of a 2-hour infusion of Ringer's solutions, 10 ml per minute, and during the last forty minutes of a one hour infusion of Ringer's solution and indomethacin, 0.35 mg per minute. Mean urinary iPGE was 4.69 plus 1.25 ng/min for the left kidney and decreased by 47 percent during Ringer's and by 83 percent during Ringer's plus indomethacin. In another series of studies 2.5 percent dextrose and water, 5 ml/min was infused throughout. Mean urinary iPGE was 3.44 plus or minus 1.23 ng/min for the left kidney and decreased by 48 percent after two hours and by 21 percent after 3 hours. Sodium excretion changed only in the studies with Ringer's. Thus, urinary iPGE exhibits circadian changes and does not appear to be influenced by expansion of extra-cellular fluid. A decrease in urinary iPGE of greater than 50 percent appears to be necessary for a change in sodium excretion.